[fuckingquest]

попробуй так

<div id = "content">1<br>2<br>3</div>
<button id = "b">сoхранить</button>

<script>

b.onclick = function(){
  
  var a = document.createElement("a")
  a.setAttribute("href", "data:text/plain," + content.innerHTML.replace(/<br>/g, encodeURIComponent("\r\n")))
  a.setAttribute("download", "filename.txt")

  a.click() 
}
</script>